1. Field of the Invention
The present invention relates to a heat-resisting metal-sheathed cable for a sensor such as an oxygen sensor or thermistor which is disposed in a high temperature place in an automobile or the like.
2. Description of the Related Art
An oxygen concentration cell type oxygen sensor is heretofore known as a sensor for detecting an air-fuel ratio of an internal combustion engine such as an automotive engine. The oxygen sensor includes a pair of porous electrodes disposed on the opposite sides of a partition wall member made of an oxygen ion conductive solid electrolyte and is constructed so as to put one of the porous electrodes into contact with the reference gas and the other into contact with the measured gas (i.e., gas to be measured) such that the oxygen concentration is measured based on the electromotive force cell across the electrodes.
Accordingly, it is important that the reference gas is accurate. For example, if gasoline, water or the like into a chamber containing the reference gas, the reference gas is contaminated resulting in inaccurate detection. For this reason, it has heretofore been practiced to provide the chamber with an air-tight wall surrounding the reference gas thereby preventing ingress of contaminants into the chamber.
However, the above described technology still has a difficulty in preventing the ingress of contaminants completely, so it has been proposed to provide the chamber with an atmospheric air introducing hole which is devised to make it difficult for the contaminants to intrude therethrough into the chamber such that the chamber can be ventilated so as to contain the normal atmospheric air even if the contaminants intrude into the chamber a little.
For example, it is disclosed in Japanese utility model provisional publication No. 56-4852 such a technology of using a covered harness having an insulation covering made of a resinous material for fetching or output of signal, providing the harness with a hollow portion at the axial center thereof and using the hollow portion as a ventilation hole for passage or conduction of the atmospheric air.
However, since the recent oxygen sensors are generally used at a high temperature, such a harness having a resinous covering encounters a problem that it cannot withstand the heat at a high temperature.
Another technology describes a sensor provided with a filter for thereby attaining the permeability. However, this technology utilizes a filter made of a resinous material and thus encounters a problem that the sensor cannot be used at high temperatures.
As a countermeasure to such a problem, it is considered to use a stainless steel (SUS)-sheathed wire having a good resistance, for the above described harness. However, since a mass of magnesia power is used in the conventional SUS-sheathed wire as an insulator, it is difficult to form such a hollow portion as in the resinous harness, in the mass of magnesia powder. For this reason, the above described countermeasure encounters a problem that a desired ventilation hole cannot be formed in such a SUS-sheathed wire.